We have previously found that voluntary exercise exerts a suppressive effect on N-nitrosomethylurea (NMU)-induced rat mammary tumor development. The purpose of this study is to investigate the inhibitory role played by voluntary exercise in the development of (a) the 7,12-dimethylbenz(a)anthracene (DMBA) induced rat mammary carcinoma, (b) spontaneously generated mouse mammary carcinoma, and (c) the metastatic spread of rat mammary tumor cells from a primary implant. Quantitative assessment of voluntary exercise will be by a specially designed wheel cage unit equipped with an activity wheel to which animals have free access to and egress from. The experimental diets are designed to mimic the high-fat (HF) western diet (40% of calories as fat) and a low-fat (LF) diet (20% of calories as fat), typical of low risk countries such as Japan. The experimental protocols are designed to test the effects of voluntary exercise on the promotion phase (DMBA-model using the NMU model as a positive control), overall tumorigenesis in a virally- induced (C3H) mouse model and the progression (or metastatic) phase (R13762 transplantable mammary tumor model). For each dietary condition, there will be a sedentary and an active group. At termination, tumor latency, frequency, multiplicity and size will be assessed in the DMBA and C3H models. Tumor size and total metastatic tumor burden will be assessed in the R13762 model, and the inhibitory effects of exercise determined by comparing active animals with their sedentary counterparts. As a means to explore mechanisms, at termination, serum prolactin (PRL) and growth hormone (GH) levels in the active and sedentary groups will be assayed and compared. Circulating GH will be assayed by standard DIA procedures. Prolactin, a known mammary tumor promoter, will be measured by two different methods: the conventional immunoassay and the No2 rat lymphoma bioassay. The No2 assay, which tests the mitogenic rather than the immunogenic properties of prolactin provides a means to determine whether novel (less mitogenic?) molecular forms of PRL are released under conditions of moderate to low-intensity exercise. The overall goal of this study is to determine whether the protective effect of voluntary exercise is limited to a particular species, strain, initiating agent or stage of carcinogenesis, or whether it can be extended to encompass mammary carcinogenesis in general and ultimately human breast cancer.